Waxing and waning of dynamical heterogeneity in the superionic state

Abstract

Using molecular dynamics simulations of UO2-a type II superionic conductor-we identify a well-defined onset of dynamic disorder (Tα), which is remarkably correlated to a nontrivial advance of dynamical heterogeneity (DH). Quantified by the correlations in the dynamic propensity and van Hove self-correlation function, the DH is shown to grow with increasing temperature from Tα, peak at an intermediate temperature between Tα and Tλ-the superionic transition temperature-and then recede. Surprisingly, the DH attributes are not uniform across the temperatures-our investigation shows a low temperature (αT) stage DH, which is characterized by weak correlations and a plateaulike period in the correlations of the propensity, and a high temperature (λT) stage DH with strong correlations that are analogous to those in typical supercooled liquids. Our work, which has rigorously identified the onset of superionicity, gives a different direction for interpreting scattering experiments on the basis of statistical, correlated dynamics.

Ion Hopping and Constrained Li Diffusion Pathways in the Superionic State of Antifluorite Li2O

“…For the Ewald sum calculations, a relative error of 10 −7 is assigned (for forces), while for the Wolf method, the damping parameter (α) is chosen as 0.3. Our earlier simulations [13,[21][22][23] on superionic UO 2 and CaF 2 also showed excellent agreement between these methods.…”

“…However, the decrease in magnitude with temperature suggests perceptible disorder among Li ions while preserving a crystalline sub-lattice, which is somewhat counter-intuitive. One possible way to accommodate disorder among Li ions within the Li sub-lattice is through dynamic hopping of Li ions between lattice sites, a mechanism that has been observed in prior atomistic simulations of Li 2 O [26,27], and ubiquitous among the anions of fluorite superionic conductors in the superionic state [13,21,23]. We will demonstrate later that such cation hopping between native sites indeed takes place and Li ions diffuse through string-like dynamical structures.…”

“…The neutron diffraction measurements give quantitative information on disorder by measuring the fraction of Li ions that are displaced from their native lattice sites at different temperatures, as shown in Figure 2a. Using the best fit to the diffraction data (shown by the dotted line), the onset of disorder can be estimated to occur at Tα ≈ 1000 K. In our earlier study, we had established that fluorite conductors such as PbF2, SrCl2, and CaF2 show a rapid rise in the ionic conductivity at Tα though with a higher activation energy [16,21]. As shown in Figure 2b, the ionic conductivity of Li2O broadly falls into two Arrhenius segments with a noticeable change in slope at Tα ≈ 1000 K. Both neutron scattering and ionic conductivity measurements, therefore, establish that the onset of Li disorder takes place at the characteristic temperature Tα ≈ 1000 K. [10].…”

“…Atomistic simulations have further helped in elucidating the structural and dynamical changes observed across Tα [13,[21][22][23], as well as across the λ transition. We thus regard a Type II ionic conductor to be in the superionic state at temperatures above Tα.…”

Low Dimensional String-like Relaxation Underpins Superionic Conduction in Fluorites and Related Structures

“…In the Gillan picture, ion conduction is emanating from the ion hopping process itself albeit without a satisfactory exposition of the specifics of the jump behavior. The current analysis, which advances our recent work1516 on fluorites that was motivated by the seminal insights elucidated by Gray-Weale and Madden1718 on heterogeneous cooperative dynamics among the mobile anions, is directed at resolving this long-standing puzzle. Rather than appealing to the traditional defect hypothesis, we show, using atomistic simulations, that relaxation of low dimensional string-like structures governs the dramatic rise in ionic conductivity spanning several orders in magnitude.…”